In a typical resin transfer molding process, a fiber preform of the article to be molded is placed within a mold cavity, and a liquid resin is introduced into the mold cavity. The resin is absorbed by the fiber preform, and then thermally cured to form the article of manufacture. Typical fibers used to make fiber preforms include fiberglass, graphite, carbon and Kevlar, and the fibers are often braided or woven into a sheet form. The resins are typically epoxy resins, although other types of resins, such as polyester resins, are also employed.
In a typical prior art system for resin transfer molding, the liquid resin is introduced into a heated pressure pot, which is coupled in fluid communication by a resin line to the mold cavity of a steel mold. The mold cavity is in turn coupled in fluid communication to a collection pot, which is coupled to a vacuum pump. The resin is typically heated to mold temperature, i.e., catalyzed, in the pressure pot. The fiber or structural preform is placed in the mold cavity, and a negative pressure is created in the mold cavity by the vacuum pump prior to introduction of the liquid resin into the mold cavity. The liquid resin is transferred from the pressure pot to the mold cavity by introducing pressurized gas into the pressure pot, which in turn causes the pressurized resin to flow into the mold cavity. As the pressurized, heated resin is introduced into the mold cavity, it is absorbed by the fiber preform.
At selected time intervals after introduction of the heated resin into the mold cavity, an operator opens the line coupled between the collection pot and the mold cavity and bleeds resin from the mold cavity into the collection pot. The collection pot typically includes a sight glass to enable the operator to view the released resin, and determine if there are air bubbles visible in the resin. The operator then determines, based on experience and skill, whether the mold cavity is filled with resin (i.e., whether the fiber preform is saturated with resin). If there are visible air bubbles, the operator closes the line to the collection pot, pressurizes the pressure pot again, and continues to introduce pressurized resin into the mold cavity. This resin bleeding and inspection process is repeated at spaced intervals in time until the operator determines based on experience and skill that the mold cavity is purged of air, and is filled so that the fiber preform is saturated with resin.
Each resin bleeding and inspection step is relatively time consuming, primarily because after the operator bleeds resin into the collection pot, the pressure pot must be pressurized again. This bleeding and inspection step is typically repeated at least three or four times for each article that is molded, making the resin transfer process time consuming and relatively expensive. Resin is also wasted each time the operator is required to bleed the resin into the collection pot to inspect the resin. Because this occurs several times during the molding of each article, the volume of wasted resin and corresponding wasted costs can be substantial.
Another drawback of such prior systems is that they rely on operator judgment to determine when the mold cavity and fiber preform are filled with resin. This typically results in poor repeatability, and lower overall quality of the articles being produced. If the operator prematurely terminates the resin transfer molding process, the molded part will typically have voids caused by an insufficient absorption of resin by the structural preform, which usually renders the part not usable. This is particularly the case in the aerospace industry, wherein voids or other such defects in aircraft components is intolerable.
It is an object of the present invention to overcome the drawbacks and disadvantages of such prior art apparatus and methods for resin transfer molding.